Microporous films may be prepared by thermally induced phase separation (TIPS) processes. The noted processes typically involves extruding materials onto a casting surface. For example, the cast film may be a precursor to a microporous film where a molten solution of at least one crystallizable polymer (e.g., polyethylene) and a miscible diluent compound (such as mineral oil) is extruded from the die at a higher temperature (example: 400xc2x0 F.) onto a lower temperature casting roll (example 150xc2x0 F.). The resulting homogeneous solution phase separates when cooled after extrusion on the casting surface to form a two phase film of polymer and compound. The miscible compound (e.g., mineral oil) can be removed in subsequent processes such as solvent extraction etc. The resulting film can also be tentered and converted in subsequent processes to form a network of interconnected pores. The microporous materials of the present invention can be employed in a wide variety of situations where their microporous structures are useful. Microporous articles may be free-standing films or may comprise structures that have microporous, layers of the invention affixed to a substrate, such as structures made from materials that are polymeric, woven, nonwoven, foil or foam, or a combination thereof depending on the application. For example, they may be used in such diverse applications as the ultra filtration of colloidal matter, as diffusion barriers, as face oil removers, as diffuse light reflectors, or as separators in electrochemical cells. Further, they may be laminated to various substrates and the laminate may be utilized to form such articles as raincoats or other outerwear or camping equipment such as tents and sleeping bags. The microporous sheets of the present invention can be laminated to a woven cloth or a non-woven fabric such as a non-woven scrim. This scrim may be used to produce a disposable protective garment for use in a hospital or in an electronic clean room or in other areas such as where caustic chemical spills may be a problem.
The microporous sheet materials may be further utilized as filtering materials for cleaning antibiotics, beer, oils, bacteriological broths, for sample collection in air analysis, and for collecting microbiological specimens. They may also be utilized to make surgical dressings, bandages, and in other medical applications. Those of ordinary skill in the art will recognize that there are many other uses for microporous materials made in accordance with the present invention. See, for example, co-assigned U.S. Pat. Nos. 4,726,989 and 4,539,256 which are hereby incorporated herein by reference in their entirety.
In this process, it has been discovered that the quality of the resultant microporous film may be degraded by several mechanisms. First, the uniform quenching of the extruded film onto the casting roll is used to form a defect free film product. If the pinning wire is contaminated or damaged, uneven charging of the film will result, causing defects such as down web lines known as xe2x80x9cwormsxe2x80x9d.
It is conventionally recognized in the art of film casting to provide a pinning wire that is electrostatically charged to force the extruded material onto the casting surface. In one method, the pinning wire continuously traverses across the film width to present a fresh pinning wire which results in defect free surface on the extruded material. However, in the instance of microporous film manufacture, the diluent phase (ex. mineral oil), even though possessing a low volatility at room temperature, is volatile enough at the extruded high temperature to flash-off or evaporate a fractional amount. This flash-off of the low volatility solvent (typically mineral oil etc.) condenses on the pinning wire at a high enough rate and amount that the xe2x80x9cwormxe2x80x9d defects may be formed.
Second, the macroscopic appearance of the microporous film is adversely affected by the environment as found in conventional film extrusion heads. The airflow in these heads is either uncontrolled room type air or worse, even higher flows such as from exhaust ducts and plenums provided for ventilation or even partial capture of the flash-off low volatility diluent. This airflow and resultant uneven heat transfer can cause large scale visible patterns in the final film reminiscent of the xe2x80x9cwood grainxe2x80x9d or xe2x80x9cmottlexe2x80x9d patterns observed in dried coatings. This is the result of uneven conditions for the phase separation or quenching process that occurs in the extruded film. Additionally, the uneven conditions often lead to undesirable variations in the microscopic pore characteristics.
Finally, for continuous running of the microporous film casting process, it is an undesirable consequence that the fugitive low volatility solvent (e.g. mineral oil) that flashes off of the film recondenses on and contaminates the equipment and facility in the vicinity of the process. The condensate from a low volatility evaporative component can also contaminate the final microporous film product. Also, some of this oil/solvent is inevitably released into the air and the environment.
In this invention, deficiencies of microporous film casting in thermally induced phase separation processes are addressed by providing a controlled environment over a film cast onto a casting roll. A controlled environment enables the reduction of defects and the improved control over the process equipment and process conditions that lead to defects. The controlled environment is created through the use of capillary condensing surface technology. The combination of capillary condensing surface technology with thermally induced phase separation processes provides an improved method for forming microporous films.
The present invention is a method for producing microporous films. The method utilizes a casting surface and a condensing surface. The condensing surface is spaced from the casting surface to form a gap. The distance between the surfaces forming the gap is relatively small and is preferably less than 3 cm. A material is cast onto the casting surface, preferably through conventional extrusion practices. The material is capable of forming microporous films by thermally induced phase separation.
The casting surface in the present invention is in motion while the material is cast onto the casting surface. The casting surface is moved in a direction relative to the condensing surface in order to move the material through the gap. The material contains at least one evaporative component that generally flashes off upon formation of the microporous material and condenses on the condensing surface. The condensing surface temperature may be controlled to adjust the condensation rate of at least one evaporative component in the material.
The method is suitable for forming microporous films without defects associated with conventional processes related to the condensation of the evaporative component. In a preferred embodiment, the method is suitable for microporous free-standing films or structures that have microporous layers of the invention affixed to a substrate, such as structures made from materials that are polymeric, woven, nonwoven, foil or foam, or a combination thereof depending on the application. For example, they may be used in such diverse applications as the ultra filtration of colloidal matter, as diffusion barriers, as face oil removers, as diffuse light reflectors, or as separators in electrochemical cells. Further, they may be laminated to various substrates and the laminate may be utilized to form such articles as raincoats or other outerwear or camping equipment such as tents and sleeping bags. The microporous sheets of the present invention can be laminated to a woven cloth or a non-woven fabric such as a non-woven scrim. This scrim may be used to produce a disposable protective garment for use in a hospital or in an electronic clean room or in other areas such as where caustic chemical spills may be a problem.
The microporous sheet materials may be further utilized as filtering materials for cleaning antibiotics, beer, oils, bacteriological broths, for sample collection in air analysis, and for collecting microbiological specimens. They may also be utilized to make surgical dressings, bandages, and in other medical applications. Those of ordinary skill in the art will recognize that there are many other uses for microporous materials made in accordance with the present invention.
The present invention also includes an apparatus for making microporous films through thermally induced phase separation processes. The apparatus includes a casting surface that is suitable for receiving a cast material. The material capable of forming microporous films by thermally induced phase separation. In general the material includes at least one evaporative component upon formation of said microporous film. A condensing surface is spaced from the casting surface to form a gap between the casting surface and the condensing surface. The casting surface is capable of moving in a direction relative to said condensing surface in order to move the cast material through the gap. The condensing surface is maintained at a temperature to condense at least a portion of the at least one evaporative component on the condensing surface.
For purposes of the present invention, the following terms used in this application are defined as follows:
xe2x80x9cmicroporousxe2x80x9d means product or material characterized by a multiplicity of spaced, randomly disposed, non-uniform shaped, equiaxed particles of a crystallizable thermoplastic polymer; and
xe2x80x9cevaporative componentxe2x80x9d means a component or ingredient present as a major phase (diluent) in the film formation step or as an additional ingredient that is volatile at the casting temperatures.
Other features and advantages will be apparent from the following description of the embodiments thereof, and from the claims.